A cement retarder is categorized into a cement admixture (a cement additive) which is admixed with cement for the purpose of retardation of setting and hardening of mortar or concrete by means of retarding hydration of cement. With respect to a cement retarder, Japanese Patent Application Laid-open No. 172250/1989 (JP-A-1-172250) mentions lead oxide, boron oxide, borax, zinc chloride, zinc oxide, magnesium silicofluoride and other inorganic retarders, polyhydroxy compounds (sugar, an alcohol, methyl cellulose, ethyl cellulose, poly(vinyl alcohol), dextrin, etc.), poly(sodium acrylate), a hydroxycarboxylate, a lignosulfonate, gluconic acid or its salt, pyruvic acid, .alpha.-ketoglutaric acid or other keto acids, and other organic retarders. The purpose of the use of these cement retarders is to transport a ready-mixed concrete over a long period with suppressing hardening of the concrete in summer, and to mitigate a stress due to a temperature (heat) in a large-sized concrete structure.
The surfaces of molded articles and various buildings made from concrete are practically decorated with the use of aggregate exposed finish by washing (washing finish) technology which is categorized into plastering technologies. The washing finish technology is a technology for exposing a part of aggregate by washing immediately before hardening of the concrete and hence washing out mortar of the surface layer of the concrete. According to this technology, the washing should be conducted in an extremely limited period (timing) since opportunity for washing closely relates to the hardening rate (cure rate) of the concrete. Therefore, the technology is unsuitable for commercial production or large-sized building applications.
With respect to execution of tile works to a surface of concrete, a technology using a unit tile as produced by disposing and adhering plural of tiles on a surface of a tacky adhesive layer of a tacky adhesive tape has been proposed. According to this technology using the unit tile, tiles are applied by disposing the unit tile in a form or shuttering to which concrete is placed or deposited, pouring mortar into the form and curing the same, removing the hardened product from the form and taking off the adhesive sheet to expose the tiles on the surface of the concrete, and thereafter removing the cement reached and cured on the surfaces of the tiles to conduct surface finish. In the surface finish, the cured cement adhered to the tiles may practically be scraped by hand and this is an essential step for tile application (tile work) which is characterized in a beautiful and tiny surface. The work for removing the hardened cement adhered to tiles, however, takes much time and trouble and is complicated so that the cost for tile application increases and the removing treatment for hardened cement would cause damage or defect of surfaces of the tiles.
For the purpose of solving these problems, technologies have been proposed, which technologies comprise processing a surface of a concrete product or forming a pattern on the surface by inhibition of hardening of only the surface layer but no other part of the concrete with the application of a cement retarder which is categorized into cement additives. By way of illustration, a process has been proposed, which process comprises impregnating a retarder (hardening retarder) in paper, fixing the impregnated paper on the bottom of a form by means of applying or posting, depositing mortar into the form, and after hardening of the concrete, removing the concrete molded article from the form, and washing fresh mortar on the surface of the concrete which is in contact with the impregnated paper to expose aggregate and hence to obtain a natural feeling. Japanese Patent Application Laid-open No. 216703/1988 (JP-A-63-216703) discloses a process for forming a predetermined convex-concave pattern (three-dimensional pattern) on a surface of a concrete product, which process comprises a step of cutting a pattern paper as produced by impregnating a retarder into a paper to produce a cutting pattern paper corresponding to a predetermined letter or figure or other pattern, a step of applying the cutting pattern paper to a predetermined part of the inner surface of a form, a step of depositing mortar into the form and curing the mortar, a step of breaking up the form to take the resultant concrete product off and a step of washing a part of the concrete product corresponding to the pattern paper. Japanese Patent Application Laid-open No. 202803/1986 (JP-A-61-202803) discloses a process which comprises applying tiles or others to a paper coated with the above-mentioned cement retarder, setting or placing the tile-applied paper into a form and washing out the product in the same manner as above.
Japanese Patent Application Laid-open Nos. 38711/1993 (JP-A-5-38711) and 50411/1993 (JP-A-5-50411) disclose a process which comprises applying a mixture containing an ultra-retarder, a tackifier, an extender or filler and a white pigment to an inner surface of a form, depositing mortar or concrete into the form, hardening the mortar or concrete and taking off the cured product from the form, washing cement paste from the surface of the cement product which is in contact with the form to finish the surface of the cement product.
However, a conventional retarder which inhibits hydration mechanism of cement has high solubility with respect to water, and hence is dissolved in water or moisture in a cement concrete. Therefore, the retarder dissolves accompanied with placement of mortar or concrete, and flows in the form or the surface of a molded article or structure together with an excessive bleed water in the mortar or concrete. Accordingly, the retarder may flow into a portion where washing finish is not required or it may concentrate locally, and, to the contrary, the retarder may flow out from a portion where washing finish or decorating is required. Further, the depth of washing is particularly increased in a surface where bleeding water has passed through. Therefore, a decorative letter or figure, in particular a complicated decoration or ornament, can hardly be formed with high accuracy in a predetermined part of a surface of a concrete molded article or concrete building.
Japanese Patent Application Laid-open No. 224953/1991 (JP-A-3-224953) discloses a process for manufacturing a concrete block integrated with an ornament material (decorative material), which process comprises applying a concrete-hardening inhibitor (concrete non-hardener) to an inner surface of a form (shuttering), disposing an ornament such as a tile or stone through a joint rod, composing a reinforcing steel (reinforcement) up to a predetermined height, depositing concrete and hardening the concrete, removing the hardened concrete from the form, and washing the surface of the resultant concrete product with water. This literature discloses that the concrete non-harder is incorporated into a composition comprising an alkali swelling agent, a highly-water-absorbable polymer, a water-absorbable monomer and a curing agent to cure the composition, and thereby a highly-water-absorbable polymer-containing cured film is obtained.
Japanese Patent Application Laid-open No. 175003/1991 (JP-A-3-175003) discloses a process for manufacturing a concrete product decorated in a surface thereof, which process comprises temporally adhering a decorative member, through an non-cured coating layer, to an inner surface of a form (mold), depositing or placing concrete to harden the concrete, and thereafter removing the hardened product from the form, and taking off the non-cured coating composition on the surface and uncured portion of concrete of the resultant concrete product by washing with water. This literature mentions that the non-cured coating layer is produced with the use of a mixture of a polyester, poly(vinyl alcohol), poly(vinyl acetal) or other alkali swelling agents, and a highly-water-absorbable polymer.
According to the technologies described in these literatures, the place of ornament disposed in the form is sifted or the concrete non-curing agent flows out so that washing finished surface or pattern can hardly be formed in a predetermined portion of a concrete product. Further, since it is necessary to apply the non-curing coating composition to the surface of an ornament, improved workability and productivity of the concrete product would not be expected. Incidentally, Japanese Patent Application Laid-open No. 175003/1991 (JP-A-3-175003) mentions that the concrete non-curing agent may be in the sheet form. However, the concrete non-curing agent is composed of a mixture of the alkali swelling agent and highly-water-absorbable polymer so that the agent can hardly be molded into a sheet. Furthermore, a highly-water-absorbable polymer is required in addition to the alkali swelling agent in order to retard hardening (curing) of cement, and there is no detail disclosure concerning the polyester described as an example of the alkali swelling agent.
It is, therefore, an object of the present invention to provide a cement retarder having a high inhibiting property with respect to hardening of cement, despite its poor solubility in water.
It is another object of the invention to provide a cement retarder which is useful for inhibiting flow out of the retarder due to moisture, and forming a pattern or washing finished surface with high accuracy on the surface of a concrete product.
A further object of the invention is to provide a retardative sheet which insures inhibition of fluidization of a retarder when a mortar composition is placed, and provides uniform inhibition of hardening of the mortar in the contact surface with the mortar composition and hence insures accurate formation of a pattern or washing finished surface on a surface of a concrete product.
It is a yet another object of the invention to provide a sheet which is useful for decorating a surface of concrete with an ornament or others while inhibiting damage of the surface and hence for providing a dress finished concrete product.
A still further object of the invention is to provide a retardative sheet which insures easy and efficient formation of a pattern or washing-finished surface on a surface of a concrete product.
It is an another object of the invention to provide a retardative sheet which insures easy formation of a complicated pattern on a surface of a concrete product and hence provides improved decorating quality.